In addition to ovid's tutorial already mentioned, here are a few more links for you to check out:
Perl CGI Tutorial
Beginner's Guide to CGI Scripting with Perl

Also, you'll need to pay close attention to security if this is for an ecommerce app. E.g. don't trust that the order total passed to you in a POST is right, as it's easy for an evildoer to make up their own price and bypass your validation. One way around this is to store sensitive data on the server, and reference it via a session id, which gets passed via POST or GET. Have a look at Apache::Session. You can also validate data being passed to you by including an MD5 (or SHA) hash of the values plus some secret key.